1. Field of the Invention
The present invention relates to photosensitive thick film compositions to be patterned by a photolithography method and to electronic devices formed by using the photosensitive thick film composition, and more particularly, relates to a photosensitive thick film composition comprising a photosensitive organic component containing an organic binder having an acidic functional group and an inorganic component containing a multivalent metal compound.
2. Description of the Related Art
Recently, the performance of various electronic apparatuses, such as mobile information terminals, and computers, have significantly improved, and in particular, greater information processing and the miniaturization of electronic apparatuses have accelerated. In addition, miniaturization and higher densities in electronic devices, which have been primarily responsible for improvements in electronic apparatuses, are also required, and three-dimensional conductive patterns for electronic devices are progressing.
In laminated electronic devices having three-dimensional conductive patterns therein, via holes are generally provided so that the conductive patterns which are disposed between a plurality of insulating layers are connected three-dimensionally with each other.
In laminated electronic devices formed by so-called thick film printing, via holes are formed by steps of (1) preparing an insulating paste by mixing powdered glass with an organic vehicle, (2) printing the insulating paste on a substrate so as to form a coating film having holes therein used as via holes, (3) filling a conductive paste in the holes used as via holes, and (4) baking the coating film and the conductive paste.
In this type of laminated electronic device, the patterning of holes used as via holes is generally performed by screen printing of an insulating paste. Accordingly, blurring and faintness are likely to occur on the coating film after screen printing caused by inappropriate viscosity of the paste, course screen mesh, and the like, and holes approximately of 100 μm or less in diameter used as via holes are difficult to form.
For example, a method is disclosed in Japanese Unexamined Patent Application Publication No. 6-283846 in which the desired via holes are formed by a step of preparing a photosensitive insulating paste by adding a photosensitive organic component, such as a photopolymerizable monomer and a photopolymerization initiator, to an insulating paste primarily composed of glass, mixing the paste thus prepared, coating the insulating paste thus formed on a substrate, and forming the desired via holes by exposing and developing. Since a photolithographic method using a photosensitive insulating paste is a patterning method using exposing and developing, fine via holes can be formed with a high degree of accuracy.
Recently, it is desired in a photolithographic method using a photosensitive insulating paste or a photosensitive conductive paste that development be performed using water or an aqueous alkaline solution in consideration of environmental conservation. In response to this desire, an organic binder having acidic functional groups, such as carboxyl groups, is used in a photosensitive organic component.
However, when glass such as SiO2—Bi2O3—B2O3 glass is used as an insulating component in a photosensitive insulating paste, multivalent metal ions of the multivalent metal oxide in the glass may dissolve in the solute of the paste in some cases. In such cases, the multivalent metal ions react with acidic functional groups (anions of the organic binder) so as to form a three-dimensional network in the paste by ionic cross-linking, and as a result, the paste itself is gelled. When the photosensitive insulating paste is gelled, the coating step is difficult to perform, and even if the coating can be performed, reliable development thereof is difficult to achieve.
In electronic devices primarily for use in various electronic apparatuses, such as mobile information terminals and computers, concomitant with greater information processing, higher signal processing speed (in particular, higher frequency) are required. In response to this requirement, the conductive pattern for transmitting signals is required to have a finer design (i.e., less width) yet a thicker film.
In general, a conductive pattern is formed by a method in which a powdered conductive metal, such as silver or copper, is added to and mixed with an organic vehicle so as to prepare a conductive paste, the desired conductive pattern is formed by the conductive paste on a non-baked or a baked insulating substrate, and after drying, the conductive pattern on the substrate is then baked. The formation of conductive patterns on insulating substrates is generally performed by a screen printing method using a screening mesh, and in such a method, blurring and faintness of the pattern which are caused by inappropriate viscosity of the paste, coarse mesh size, and the like, are difficult to avoid. Hence, a pattern having wire widths and wire spaces of approximately 50 μm, respectively, is the limit of the screen printing method.
The formation of a fine and thick film wiring by a photolithographic method using a photosensitive conductive paste is disclosed in Japanese Unexamined Patent Applications Publication Nos. 5-287221 and 8-227153. In this method, a photosensitive conductive paste which is composed of a powdered conductive metal, a photosensitive organic component formed of an acrylic copolymer, a photoreactive compound and a photopolymerization initiator, is coated on the entire surface of an insulating substrate, and after drying, the desired conductive pattern is then formed by a photolithographic method.
When a conductive pattern is formed on a ceramic substrate using a photosensitive conductive paste, an inorganic additive component, such as PbO—B2O3—SiO2 glass or SiO2—Bi2O3—B2O3 glass, may be added to and mixed with the photosensitive conductive paste in order to improve the adhesive strength between the conductive pattern and the ceramic substrate (refer to Japanese Unexamined Patent Applications Publication Nos. 6-224538, 8-335757). In consideration of environmental conservation, the development of the photosensitive conductive paste is desired to be performed using water or an aqueous alkaline solution. As is the case with the photosensitive insulating paste, an organic binder having an acidic functional group, such as a carboxyl group, is used in the photosensitive organic component.
However, as is also the case with the photosensitive insulating paste, multivalent metal ions of the inorganic additive component (such as powdered glass) contained in a photosensitive conductive paste dissolve in the solution for the photosensitive conductive paste and react with the acidic functional groups of the organic binder, forming a three-dimensional network in the paste by ionic cross-linking, and as a result, the paste itself is gelled.
Thus, in a photosensitive conductive paste composed of an inorganic additive component having multivalent metal ions capable of being dissolved and a photosensitive organic component containing an organic binder having an acidic functional group, the formation of a three-dimensional network by ionic cross-linking is likely to occur. As a result, the photosensitive conductive paste is easily gelled, and when the gelation progresses, coating is difficult to perform, or even if the coating can be performed, reliable development of the pattern is difficult to perform.
As methods for preventing gelation of photosensitive pastes, such as photosensitive insulating pastes and photosensitive conductive pastes, for example, a method is disclosed in Japanese Unexamined Patent Application Publication No. 9-218509 in which a compound containing a phosphorous compound, such as phosphoric acid, is present, a method is disclosed in Japanese Unexamined Patent Application Publication No. 9-218508 in which a compound having an azole structure, such as benzotriazole, is disclosed, and a method is disclosed in Japanese Unexamined Patent Application Publication No. 9-222723 in which an organic compound having a carboxyl group, such as acetic acid, is present. However, the methods mentioned above only prolong the time for gelation of the photosensitive paste to some extent.
In Japanese Unexamined Patent Application Publication No. 10-171107, a method is disclosed in which 3-methyl-3-methoxybutanol is present in a photosensitive paste. In this method, however, even though the gelation of the paste itself can be prevented, a phenomenon similar to gelation is observed in the coating film after drying, that is, a three-dimensional network by ionic cross-linking is found, and as a result, the molecular weight of the film is substantially increased. Accordingly, non-exposed areas of the film are difficult to dissolve in the developing solution during development.